This invention relates to a semiconductor thin film producing apparatus used in a crystallizing process of crystallizing a semiconductor material and, in particular, to a laser annealing apparatus used in a production process of a liquid crystal display or a contact image sensor.
In a crystallizing process of crystallizing an amorphous silicon film deposited on a substrate (for example, a glass substrate), use has widely been made of a laser annealing technique utilizing an excimer laser. This is because, by the use of the excimer laser, the crystallizing process can be performed with the substrate kept at a relatively low temperature. The excimer laser produces a laser beam having high energy sufficient to melt and crystallize amorphous silicon. The laser beam produced by the excimer laser has a very small pulse width on the order of several tens nanoseconds so that crystallization is quickly completed within a time period on the order of one hundred nanoseconds. Therefore, the heat produced by the laser beam is not transferred to the substrate. Thus, the crystallizing process can be carried out without exposing the substrate to a high temperature.
Referring to FIG. 1, a conventional laser annealing technique uses a laser beam LO having a beam profile illustrated in the figure. The laser beam LO is called a long beam or a line beam. By the use of the laser beam LO continuously irradiated in a scanning direction S, an amorphous silicon film 101 deposited on an insulating substrate 102 is crystallized into a polycrystalline silicon film 103. However, the polycrystalline silicon film 103 produced in the above-mentioned manner is composed of crystal grains having a very small grain size on the order of several hundreds nanometers at maximum. Therefore, a large number of grain boundaries are present in the polycrystalline silicon film 103. Since the grain boundaries prevent the conduction of charge carriers, presence of such a large number of grain boundaries reduces the mobility of thin film transistors (TFT). As a result, it is impossible to produce an active matrix liquid crystal display (AMLCD) having a high response speed.
In view of the above, it is desired to produce a polycrystalline silicon film composed of crystal grains of a large size with a reduced number of grain boundaries. To this end, a crystallizing process using a projection mask is proposed by James S. Im et al in Applied Physics Letters 69(19), Nov. 4, 1996, p. 2864.
Referring to FIG. 2, the Im et al crystallizing process will be described.
The im et al crystallizing process utilizing a laser annealing technique is implemented by an apparatus illustrated in FIG. 2. A laser oscillator 10 emits a laser beam LO. The laser beam LO is attenuated by an attenuator 110 and reflected by first and second total reflection mirrors 11-1 and 11-2. After passing through a field lens 111, the laser beam LO is shaped by a projection mask 18 into a narrow beam LN having a beam width on the order of 5 xcexcm in accordance with a mask pattern formed on the projection mask 18. The narrow beam LN passes through an image-forming lens 112 and reflected by a third total reflection lens 11-3 to be irradiated onto a substrate 16 supported on a substrate stage 17. On the substrate 16, an amorphous silicon film (not shown) is preliminarily deposited. After a single step of irradiation, the substrate 16 is displaced by moving the substrate stage 17 and another step of irradiation is carried out. By repeating the above-mentioned operation, the substrate 16 is scanned throughout its entire surface and the crystal growth takes place in the amorphous silicon film deposited on the substrate 16. Herein, the substrate 16 is scanned at a scanning pitch on the order of 0.75 xcexcm.
In the Im et al crystallizing process described above, the distance of the crystal growth in each single step of laser irradiation is so short that the substrate must be scanned at such a narrow pitch. Every time when the substrate is scanned, the laser beam is irradiated with high energy sufficient to melt amorphous silicon. This imposes a heavy load upon the projection mask. Upon irradiation of the laser beam, the projection mask absorbs the energy of the laser beam to be heated to an extremely high temperature. This results in occurrence of distortion of the mask pattern and ablation of a film. In this event, the projection mask can not be used any longer. In order to avoid the above-mentioned problem, the projection mask must be made of a material having a high reflectance so as not to substantially absorb the energy of the laser beam.
In a current lithography process for producing a large scale integrated circuit (LSI), use is often made of a stepper or an exposure apparatus comprising an excimer laser such as KrF and ArF as a light source. Generally, the stepper uses a reticle made of Cr as a mask material. The reflectance of Cr with respect to an ultraviolet ray is as low as about 60%. Therefore, such reticle can be used in the stepper without any disadvantage as far as the laser beam has low energy per pulse. However, in the above-mentioned crystallizing process using a high-energy laser beam, such reticle is not applicable because heat generation resulting from energy absorption is seriously great.
It is therefore an object of this invention to provide a semiconductor thin film producing apparatus for irradiating a semiconductor thin film with a laser beam corresponding to an aperture pattern formed in a mask to reform the semiconductor thin film, in which the mask is enhanced in durability and prolonged in life to reduce the frequency of exchange from one mask to another mask so that a high productivity is achieved.
This invention achieves a mask enhanced in durability and prolonged in life to reduce the frequency of exchange from one mask to another mask in a process of reforming a semiconductor thin film by a large number of times of irradiation of a high-energy excimer laser beam.
According to a first aspect of this invention, there is provided a semiconductor thin film producing apparatus for irradiating a semiconductor thin film by a laser beam through an aperture pattern formed in a mask to reform the semiconductor thin film, wherein the mask has a reflecting surface having a reflectance not smaller than 70% with respect to the laser beam.
Thus, it is possible to achieve a mask highly durable against a high-energy excimer laser beam.
According to a second aspect of this invention, the apparatus further comprises a cooling mechanism for cooling the mask.
By cooling the mask with water or liquid nitrogen, the increase in temperature of the mask is suppressed to further enhance the durability of the mask.
According to a third aspect of this invention, the mask comprises a substrate material and a mask material formed on the substrate material. The mask material has the aperture pattern and the reflecting surface having the reflectance not smaller than 70% with respect to the laser beam. The substrate material transmits to the semiconductor thin film the laser beam which is transmitted through the aperture pattern formed in the mask material.
By depositing the mask material as a thin film on the substrate material transmitting the laser beam, it is possible to achieve a fine structure as the aperture pattern formed on the mask.
According to a fourth aspect of this invention, the mask further comprises a transmitting film formed on the substrate material with the mask material interposed between the substrate material and the transmitting film.
The transmitting film serves to protect the mask by suppressing oxidization and ablation of the mask material.
According to a fifth aspect of this invention, the semiconductor thin film producing apparatus further comprises at least one absorbing substrate arranged at a position before the laser beam is irradiated onto the mask. The above-mentioned at least one absorbing substrate is for locally absorbing the laser beam.
According to a sixth aspect of this invention, the above-mentioned at least one absorbing substrate is furthermore for absorbing a reflected beam which is reflected by the reflecting surface of the mask when the laser beam is irradiated onto the reflecting surface of the mask.
By providing the absorbing substrate for absorbing the laser beam, it is possible to suppress a damage caused by a reflected beam reflected by the mask to an optical system, for example, comprising a homogenizer
If providing a plurality of absorbing substrate for absorbing the laser beam, the energy of the laser beam absorbed by each absorbing substrate and the heat generation in each absorbing substrate are reduced to thereby enhance the durability of the absorbing substrate.
According to a seventh aspect of this invention, the semiconductor thin film producing apparatus further comprises a reflecting substrate arranged at a position before the laser beam is irradiated onto the mask to be in anti-parallel to the mask. The reflecting substrate has a reflecting surface having a reflectance of 70% or more with respect to the laser beam for locally reflecting the laser beam as a reflected beam when the laser beam is irradiated onto the reflecting surface of the reflecting substrate.
By provision of the reflecting substrate, it is possible to suppress a damage caused by a reflected light beam reflected by the mask to an optical system, for example, comprising a homogenizer.
According to an eighth aspect of this invention, the semiconductor thin film producing apparatus further comprises a mechanism for returning the reflected beam to an optical path of the laser beam.
With this structure, repeated irradiation can be carried out at time intervals without using a plurality of laser oscillators.
According to a ninth aspect of this invention, the semiconductor thin film producing apparatus further comprises an optical system arranged in a return route through which the reflected beam is returned to the optical path.
With this structure, it is possible to trim the profile of the reflected light beam.
According to a tenth aspect of this invention, the semiconductor thin film producing apparatus further comprises a mechanism for freely changing the length of a return route through which the reflected beam is returned to the optical path.
With this structure, it is possible to freely change the distance over which the reflected light beam travels before it is irradiated onto the substrate and to freely select the time required before the reflected light beam is irradiated onto the substrate.
Table 1 shows the reflectance (%) of each of an Al film and a Cr film with respect to an ultraviolet ray.
As seen from Table 1, it is understood that the Al film has a reflectance greater than 80% while the Cr film has a reflectance smaller than 70%. A high reflectance results in low absorption, i.e., less temperature increase. Therefore, it is believed that Al is higher in durability than Cr. Herein, each metal film was deposited by sputtering to the thickness of 200 nm.
The Ar film and the Cr film were subjected to an irradiation durability test in which these films were continuously irradiated by a laser beam emitted from a XeCl excimer laser and having a wavelength of 308 nm. The results are shown in Tables 2A and 2B.
In Tables 2A and 2B, symbols ◯, xe2x96xa1, xcex94, and x represent xe2x80x9cno change in appearancexe2x80x9d, xe2x80x9cslight discolorationxe2x80x9d, xe2x80x9cclear discoloration observed but not completely detached (not seen through)xe2x80x9d, and xe2x80x9cclear discoloration observed and detached (seen through)xe2x80x9d, respectively.
Table 2A and 2B show results of the irradiation durability test in which the films were subjected to irradiation of 100 times and 600,000 times, respectively.
From the results given above, it is understood that the Al film hardly causes discoloration and detachment as compared with the Cr film, and is durable up to 600,000 times of irradiation at the energy of 100 mJ/cm2. By concentrating the laser beam using a reduction projecting lens, the semiconductor film can sufficiently be reformed at the laser beam intensities shown in Tables 2A and 2B.